1. Field of the Invention
The present invention relates generally to adaptive equalizer systems and more specifically to a high-speed, digitally-controlled adaptive equalizer system for facilitating computer communications in a local area network.
2. Background Art
Equalization restores a data waveform'frequency components which are lost when the waveform propagates through data transmission channels such as cables. Thus, equalization permits the received waveform to closely resemble the originally transmitted waveform. A typical application of an equalization scheme in the data communications art is to facilitate digital computer communication among workstations in a local area network (LAN).
The magnitude of frequency loss in a received waveform depends upon the length of the data transmission channel. Longer transmission channels cause losses across all frequencies but with greater losses in high frequency signals. Thus, the farther apart two workstations are in a LAN, the more likely the received data will be: attenuated by frequency, shifted in phase (frequency dispersion), and attenuated with less signal-to-noise (S/N) due to crosstalk.
Adaptive equalizer systems determine and provide equalizations required for a received waveform to ultimately resemble the originally transmitted waveform. FIG. 1 shows a conventional adaptive equalizer system 100 in which workstation 102 transmits waveform 105 via transmission line 110 to workstation 115. Waveform 105 is typically the MLT3 three-level code signal. Transmission line 110 is typically unshielded twisted pair wiring. However, transmission line 110 may also include shielded twisted pairs, attachment unit interface (AUI) cables, copper distributed data interface (CDDI), coaxial transmission lines, or other types of wiring. Workstations 102 and 115 may also include other types of transmitters/receivers in a fast Ethernet (100 Mbps Ethernet) or 100Base-X communications network system. Additional details on CDDI (FDDI) are discussed in Fibre Distributed Data Interface (FDDI)--Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), American National Standard for Information Systems (Mar. 1, 1995) and in U.S. Pat. No. 5,305,350 issued to Budin et al. on Apr. 19, 1994, both of which are fully incorporated herein by reference thereto as if repeated verbatim immediately hereinafter.
The receiving end of transmission line 110 is connected through a data jack 120, such as an RJ45 jack, to the primary winding of a decoupling transformer 125 which decouples the received waveform 105'. The secondary winding of decoupling transformer 125 is connected to a transceiver chip 130 which includes an equalizer (gain stage) 135, a peak detector and comparator 140 and slicers 145 and 150. Conventional equalizer units are also shown and described in U.S. Pat. No. 5,115,213 issued to Eguchi on May 19, 1992; in U.S. Pat. No. 4,187,479 issued to Ishizuka on Feb. 5, 1980; in U.S. Pat. No. 4,689,805 issued to Pyhalammi et al. on Aug. 25, 1987; in U.S. Pat. No. 5,036,525 issued to Wong on Jul. 30, 1991; in U.S. Pat. No. 4,275,358 issued to Winget on Jun. 23, 1981; in U.S. Pat. No. 4,378,535 issued to Chiu et al. on Mar. 29, 1983; in U.S. Pat. No. 4,768,205 issued to Nakayama on Aug. 30, 1988; in U.S. Pat. No. 5,337,025 issued to Polhemus on Aug. 9, 1994; in U.S. Pat. No. 5,293,405 issued to Gersbach et al. on Mar. 8, 1994; in U.S. Pat. No. 4,459,698 issued to Yumoto et al. on Jul. 10, 1984; in U.S. Pat. No. 4,583,235 issued to Domer et al. on Apr. 15, 1986; in U.S. Pat. No. 4,243,956 issued to Lemoussu et al. on Jan. 6, 1981; in U.S. Pat. No. 4,961,057 issued to Ibukuro on Oct. 2, 1990; and in L. J. Giacoletto (editor), Electronics Designers' Handbook (2.sup.nd d.), McGraw-Hill Book Company, New York, N.Y. (1977). The references mentioned above are incorporated herein by reference. Peak detector circuits or methods used in adaptive equalizers are also disclosed in U.S. Pat. Nos. 5,293,405, 4,768,205, 4,592,068, 4,459,698, 4,873,700 and 5,036,525, which are incorporated by reference.
A peak reference source 155 generates a "PEAK-REFERENCE" signal having a specific amplitude equal to the pre-propagation amplitude of waveform 105 at some frequency. Peak detector 140 compares the absolute amplitude value of received waveform 105' (at a specific frequency) with the amplitude value of the PEAK-REFERENCE signal and generates an "ERROR" signal based on the difference in amplitudes of both signals. The ERROR signal propagates, via feedback loop 142 with gain stage 144, to equalizer 135, which equalizes received waveform 105' to resemble originally-transmitted waveform 105.
Slicer 145 outputs via line 160 an output signal "SLICER1," while slicer 150 outputs via line 165 an output signal "SLICER2." The SLICER1 and SLICER2 signals slice equalized waveform 105' at predetermined voltage levels and are also driven into OR gate 167 which outputs a non-return-to-zero-inverted (NRZI) signal. (FIG. 2 shows the slicing levels of the SLICER1 and SLICER2 signals in received waveform 105'.)
In a conventional adaptive equalizer system 100 with a peak detector 140, peak reference source 155 generates the appropriate ERROR signal based on the following reference ratio: the received waveform 105' will have an amplitude value of 2.+-.5% volts for a transmission line 110 of zero-meter length.
However, conventional adaptive equalizer systems 100 are typically unable to fully comply with the above-mentioned 2.+-.5% volt reference amplitude value.
Additionally, data jack 120 and decoupling transformer 125 often cause amplitude voltage loss in waveform 105, thereby also impacting the required 2.+-.5% volt reference voltage relied upon by peak reference source 155. Additionally, transformer manufacturers have been unable to fully prevent the amplitude voltage loss caused by decoupling transformers 125, partly due to variations in manufacturing processes.
Another disadvantage in conventional adaptive equalizer systems 100 is the difficulty in designing and manufacturing reliable CMOS-based peak detectors 140. This difficulty is a result of the following factors in CMOS technology: (1) lower transconductance, (2) greater offset presented to the inputs in the differential pair, (3) the presence of CMOS drift, and (4) process variations among different manufacturers. Peak detectors 140 may be reliably designed based on bipolar technology, but these would require more integrated circuit chip surface area and consume more power.
A conventional adaptive equalizer 100 has a further disadvantage in that peak detector accuracy depends on the pattern of the transmitted waveform. For example, FIG. 3 shows a dense-data patterned waveform 180 being received from transmission line 110 (see FIG. 1). A high peak signal 200 (FIG. 4) internal to peak detector 140 can be used to accurately detect high (positive) data pulses 180H of received dense-data patterned waveform 180, thereby accurately measuring the waveform amplitude. For a received sparse-data patterned waveform 205 of FIG. 4, internal high peak signal 200 decrements in a window 210 lacking high pulses (data) 205H. When high pulses 205H again appear in a window 215, the peak detector logic circuitry cannot increment high peak signal 200 to the actual peak 205HP of a high pulse 205H. Thus, conventional peak detectors may inaccurately measure the absolute amplitude value of received sparse-data patterned waveform 205.
In addition, experiments have shown that "pseudo-random test patterns" (i.e., linear feedback shift register LFSR patterns of orders 11, 15 and 23) yield different equalization levels, since the conventional adaptive equalizer may be tuned for one pattern (e.g., LFSR order 11) which is not optimal for another pattern (e.g., LFSR order 15). An LSFR order determines a waveform'"run-length" characteristic. Thus, waveforms with higher LSFR orders will contain longer run-length characteristics.
What is needed is a system and method for adaptive equalization which would overcome these problems of conventional adaptive equalizer systems with peak detectors.